The proposed studies employ the fetal and newborn lamb as experimental models for the study of thyroid system ontogenesis and thyroid physiology in the fetal and newborn periods. Included are studies of: histological and chemical maturation of the hypothalamus and the pituitary portal vascular system; maturation of hypothalamic-pituitary-thyroid secretion control systems; and maturation of pituitary thyrotropin releasing hormone (TRH) receptors, thyroid TSH receptors, tissue thyroid hormone receptors, and tissue catecholamine receptors. In addition, we plan to further investigate the mechanism(s) of the increase in iodothyronine monodeiodinase activity in the late fetal and early newborn periods. Finally we plan to study the mechanism(s) for augmentation of non-shivering thermogenesis in the newborn, including the influence of thyroid hormones and the interaction of catecholamines and thyroid hormones on tissue thermogenic mechanisms.